In constructing preforms from which optical fibers may be drawn, vapors of materials such as SiCl.sub.4, GeCl.sub.4 and POCl.sub.3 are entrained in a carrier gas such as oxygen and drawn as a vapor stream into the interior of a glass preform tube. The preform tube is rotated while a torch repeatedly traverses its length. As the vapor stream passes through the tube and encounters the band of heat adjacent the torch it reacts creating oxides which deposit on the interior surface of the tube. After numerous torch passes have formed numerous deposition layers the tube is collapsed into a solid rod-shaped preform.
During the just described procedure, commonly termed the modified chemical vapor deposition process, not all of the reaction products are deposited within the preform tube. Some rather are exhausted from the tube in a powdery form along with exiting carrier gas. Heretofore these undeposited reaction products have been conveyed from the preform tube through an exhaust tube of larger diameter than the preform tube formed as an integral extension of the preform tube. During the vapor deposition process, however, which ordinarily lasts for a number of hours, some of the powdery reaction products exhausted from the preform tube deposit on and accumulate within the exhaust tube. This accumulation forms a progressively increasing restriction to the flow of fluids and other reaction products later passed through the exhaust tube which in turn affects the pressure of the vapor stream within the preform tube itself. Very small changes in pressure and flow pattern at the exit end of the preform tube can substantially effect the deposition process as it is imperative that the vapors be delivered to the preform tube at precisely controlled mass flow rates. Thus, this progressively increasing restriction and changes in flow pattern within the exhaust tube adversely affects the deposition process since it is unpredictable and uncontrolled.
In an effort to alleviate the just described problems scraper rods have been laid within the exhaust tubes which continuously gravitate to the lower portion of the tube during its rotation. This has served to agitate and shake loose some of the reaction products that have deposited and accumulated on the interior walls of the exhaust tube thereby enabling the product to be withdrawn by the vapor stream. Periodically the scraper rod has also been manually moved about in both axial and radial directions in order to augment cleaning of the inside of the exhaust tube by scrapping the powdery product that may have accumulated at the throat area where the exhaust tube is joined to the preform tube as well as in the exhaust tube. This approach however, though providing improved results, has still not prevented reaction product from accumulating to such a degree as to affect the rate of the deposition process significantly. Even when agitated the products tend to diffuse upstream to some degree creating anomalies in the deposition layers. Furthermore, it has proven to be quite a tedious process to form the exhaust tube unitarily with the preform tube with precise axial alignment.
An additional problem associated with prior art preform tube exhaust systems arises during the collapse phase of the modified chemical vapor deposition process. During this period it is necessary to provide positive pressure within the collapsing preform tube in order to maintain roundness. Though there are other less desirable ways in which to do this, the best way has been to build up back pressure by sealing off the downstream end of the tube. This has been done by either holding the torch at the juncture of the preform and exhaust tubes until a seal is formed or by inserting a plug into the exhaust tube. This however means having to stop the lathe used in heating and rotating the tubes, manually removing the scraper rod and inserting a plug, all of which restricts manufacturing efficiency. Accordingly, it is to these problems to which the present invention is primarily directed.